Nonaqueous electrolyte batteries including a negative electrode containing a lithium metal, a lithium alloy, a lithium compound, or a carbonaceous material are expected as high energy density batteries, and intensively studied and developed. Hitherto, lithium ion batteries including a positive electrode containing LiCoO2 or LiMn2O4 as an active material, and a negative electrode containing a carbonaceous material which allows lithium ions to be inserted in and extracted from are widely used in mobile devices.
On the other hand, when the battery is installed in vehicles such as automobiles or trains, the components of the positive and negative electrodes preferably have high chemical and electrochemical stability, strength, and corrosion resistance, thereby providing high storage performance, cycle performance, and long-term reliability under high output at a high temperature (60° C. or higher) is provided. Furthermore, high performance is required in cold climate areas, and high output performance and long life performance at a low temperature (−40° C.) are desirable. From the viewpoint of improving safety performance of nonaqueous electrolytes, incombustible and nonvolatile electrolyte solutions are under development, but they are not still in actual use because they deteriorate the output properties, low temperature performance, and long life performance.
As described above, at least high temperature durability is required in order to install the lithium ion battery in the vehicle or the like. This is because poor high temperature durability makes it difficult to replace a lead storage battery mounted on the engine room of the automobile.
If the thickness of the negative electrode is decreased to increase the density for increasing the capacity in such a secondary battery, the current collector has insufficient strength, so that the battery capacity, output performance, cycle life, and reliability may be markedly limited. The decrease in the thickness of the electrode is also considered also from the viewpoint of providing high output. The particle size of the active material is large (for example, from several micrometers to tens micrometers), which makes it difficult to exploit high output. In particular, at a low temperature (−20° C. or lower), the rate of utilization of the active material is decreased, which causes difficult discharge. If the particle size of the negative electrode active material is increased in place of decreasing the thickness of the negative electrode, the interface resistance of the electrode is increased, which makes it more difficult to exploit high performance.
In the meantime, lithium iron phosphate (LixFePO4) and lithium manganese phosphate (LixMnPO4) attracts attentions as a lithium phosphorus metal compound having an olivine crystal structure as a positive electrode active material in order to improve the performance of a positive electrode, and the thermal stabilities thereof are improved. However, these positive electrode active materials have low electrical conductivity, which cause a problem in charge-and-discharge rate performance. Iron or manganese in the positive electrode active material is melted at a high temperature of 45° C. or higher, and deposited on the negative electrode, which accelerates deterioration in a cycle life. On the other hand, when a carbon material is used for the negative electrode, deterioration caused by metal lithium deposition is apt to be accelerated at a low temperature. Therefore, when the lithium ion battery including the positive electrode active materials is used in the automobile, it is necessary to subject the battery to air cooling or water cooling to keep the temperature of the battery as constant as possible, which causes an increase in a volume or weight and a cost increase of a battery pack.